Muscle weakness, injury and inflammation are important obstacles to normal locomotory function in individuals subjected to periods of muscle unloading followed by normal loading. These defects are expected to acquire increased importance in health care of an aging population, in which periods of convalescence can entail prolonged bed rest followed by attempts to regain normal ambulation. The design of optimal preventative or therapeutic treatments to minimize the muscle defects that arise upon reloading will rely upon identifying the factors that contribute to muscle damage during this reloading period. Our findings support the view that myeloid cells contribute to promoting injury and repair during muscle reloading. We hypothesize that neutrophils promote injury to reloaded muscle via free radical mediated events. Furthermore, we propose that the reduction in the expression of nitric oxide synthase (NOS) during muscle unloading renders muscle susceptible to neutrophil-mediated damage, because NO can normally protect muscle from damage by free radicals. Finally, we hypothesize that macrophages participate in muscle repair, so that interventions that affect their presence or activity in muscle can influence the repair process. In the study proposed here, we will test our hypothetical model of myeloid cell function in muscle injury and repair following modified muscle use. We will use mouse hindlimb suspension followed by reloading to induce injury and repair caused by modified muscle use, and assess the effects of depleting neutrophils or macrophages from the animals prior to reloading. We will also examine muscle injury and repair during reloading in transgenic mice that are null mutants or over-expressers of enzymes that generate specific molecules that are implicated in promoting muscle injury or protecting muscle from injury by myeloid cells. The results of the investigation proposed here will provide the first experimental data on the role of myeloid cells in muscle injury and repair following modified use. Those findings can indicate new therapeutic approaches to improving muscle function during muscle reloading following periods of reduced loading.